The sensitivity of the guaiac-impregnated card, hemoccult II, for the detection of colorectal neoplasia was tested in 213 patients undergoing elective colonoscopy over a 10-month period. Contrary to statements in standard texts as to the high sensitivity of guaiac-impregnated cards for the detection of these tumors, our results demonstrate only a 52% sensitivity for colorectal carcinoma and a 23% sensitivity for adenomatous polyps over 1.0 cm in diameter. We compare these results to other studies testing hemoccult sensitivity in the presence of colorectal neoplasia. This relatively low sensitivity rate emphasizes that the use of guaiac-impregnated cards alone does not exclude colorectal carcinoma in any patient.